The present invention relates generally to systems which calculate the effects of the atmosphere on the transmission of radio and optical beams, and more specifically to a moderate resolution propagation model (and system) of the earth's atmosphere. This model can be used for predicting atmospheric transmittance and background radiance from 0 to 50,000 cm.sup.-1 at a resolution of 2 cm.sup.-1.
The interest in atmospheric transmittance and background radiance along with the associated subject of astronomical refraction goes back to Laplace. With the advent of large telescopes and phase array radar systems, this interest has become ubiquitous, since the output signals of these systems experience attenuation due to atmospheric particles, water vapor and other gases along the viewing path.
The transmittance and radiance along a path through the atmosphere depend upon the total amount and the distribution of the absorbing and scattering species as well as the variation of pressure and temperature along the path. The integrated amount of absorber or scatterer along a path is known by various names, including "column density," "equivalent absorber amount," and "air mass." While the term "air mass" applies specifically to the total amount of gas along the path, it will be used here to refer loosely to the integrated amounts for all the different species relative to the amount for a vertical path.
The task of ascertaining atmospheric transmittance and atmospheric background radiance is alleviated, to some extent by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 5,315,513 issued to Abreu et al; PA1 U.S. Pat. No. 5,075,856 issued to Kneizys et al; PA1 U.S. Pat. No. 4,611,929 issued to Holyer; PA1 U.S. Pat. No. 4,661,907 issued to Arnone et al; and PA1 U.S. Pat. No. 4,521,861 issued to Logan et al.
The last three references disclose alternative atmospheric model systems. The first two patents are for the MODTRAN and LOWTRAN models that are improved by this invention. The closest reference is the Abreu et al patent, which uses an earlier model that lack the specific features specified below.
Perhaps the most significant of the above-cited references are the Kneizys and Abreu patents, which were filed for the LOWTRAN 7 and MODTRAN models of the atmosphere, which are discussed below. The Holyer reference discloses a satellite method for measuring sea surface temperature which utilizes LOWTRAN 5, which is a predecessor of the present invention.
In February of 1980, the Phillips Laboratory of Hanscom Air Force Base, Massachusetts developed LOWTRAN5, a Fortran computer code designed to calculate atmospheric transmittance and radiance for a given atmospheric path at low spectral resolution. The details of LOWTRAN 5 are described in a technical report by F. Kneizys et al entitled "Atmospheric Transmittance/Radiance; Computer Code LOWTRAN 5, AFGL-TR-80-0067," the disclosure of which is incorporated herein by reference. This report is available from the National Technical Information Service where it is identified as document number ADA088215.
In LOWTRAN 5, 6 and 7, the atmosphere is modeled as a set of spherically symmetric shells with discrete boundaries. The temperature, pressure, and absorber (gas and aerosol) densities are specified at the layer boundaries. Between boundaries, the temperature profile is assumed linear while the pressure and densities are assumed to follow exponential profiles.
LOWTRAN 6 was developed and described in August 1983 in a technical report entitled "Atmospheric Transmittance/Radiance; Computer Code LOWTRAN 6, AFGL-TR-83-0187," the disclosure of which is incorporated herein by reference. This report is available from the National Technical Information Service, where it is identified as document number ADA137796.
LOWTRAN 6 was an improvement over the previous model LOWTRAN 5, which assumed that the index of refraction was constant between layer boundaries. LOWTRAN 6 assumes a continuous profile for the refractive index, with an exponential profile between layer boundaries. It is more accurate than the previous models and works for all paths.
The LOWTRAN 7 model and computer code calculates atmospheric transmittance and background radiance for a given atmospheric path at low spectral resolution. This version is an extension and update of the current code, LOWTRAN 6 (and its predecessors LOWTRAN 5, LOWTRAN 4, LOWTRAN 3 and LOWTRAN 2). All the options and capabilities of the LOWTRAN 6 code have been retained, but additional refinements have been added, as described below.
The LOWTRAN 7 code calculates atmospheric transmittance, atmospheric background radiance, single scattered solar and lunar radiance, direct solar irradiance, and multiple scattered solar and thermal radiance. The spectral resolution of the model is 20 cm.sup.-1 (full width at half-maximum) in steps of 5 cm.sup.-1 from 0 to 50,000 cm.sup.-1 (0.2 um to infinity). A single parameter band model is used for molecular line absorption and the effects of molecular continuum-type absorption; molecular scattering, aerosol and hydrometer absorption and scattering are included. Refraction and earth curvature are considered in the calculation of the atmospheric slant path and attenuation amounts along the path. Representative atmospheric, aerosol, cloud, and rain models are provided in the code with options to replace them with user-provided theoretical or measured values.
But now LOWTRAN 7 has been displaced by its progeny--MODTRAN--with a current combined (AF and ONTAR) distribution to over 1000 users. The basic advantages of MODTRAN center on the improved spectral resolution and compatibility with HITRAN in its most recent issue ('92 and '95 for MODTRAN2 and 3, respectively). The most important generic message, however, is that most cases, all LOWTRAN versions should be replaced by MODTRAN.
Because MODTRAN3 is now undergoing external release in mid-1995 (alone with its ONTAR PC version), its advantages are restated here. The primary changes from older versions include: (1) a new solar irradiance, at 1 cm.sup.-1 resolution (first suggested by B.-C. Gao and developed by R. Kurucz); (2) an alternate multiple scattering algorithm based on DISORT (3) parameter statement control over the total number of atmospheric layers, facilitating user control over the degree of accuracy in replicating atmospheric structure (4) new IP and UV temperature dependent cross sections and (5) improved band model and radiance algorithms, developed and validated against LBL calculations. Subsequent modifications will be made to MODTRAN as they become available. For instance, a new measurement-based solar irradiance (from Shuttle measurements from the 1992 ATLAS campaign, expected in 1995) will be provided, along with the existing options.
Coupled with all previous descriptions of MODTRAN capabilities, MODTRAN3 calculations are demonstrating a breadth of unique applications. In a series of papers, led by the work of J.-M. Theriault, preliminary use of MODTRAN for analysis of moderate resolution interferometer data has been established. These papers did not advocate using MODTRAN alone as the forward radiance algorithm, but, rather, showed that MODTRAN could be employed to obtain the large derivative matrices used in physical inversion algorithms. Because these matrix elements are calculated from determining the sensitivity of small single layer perturbations of each detectable species upon the full path radiance, the near equivalence of the individual matrix elements to those derived analytically from LBL codes was unexpected.
This set of studies did not recommend that the iterative least squares minimization test of forward calculation vs. measurement employ MODTRAN. That recommendation can only be made after careful analyses of the impact of spectral resolution (and its accuracy) on vertical resolution and instrument signal to noise and other error sources. Such studies are currently being undertaken.
A direct outgrowth of the sensitivity of the matrix elements was the suggestion that the MODTRAN layer-specific optical properties (effective optical depth and transmittance/radiance differentials; that is, those quantities derived from differences and ratios of adjacent full path BM calculations) were essentially equivalent to the LBL direct layer calculations. Given the near linearity of this sensitivity, MODTRAN became a candidate for flux-divergence and energy deposition calculations.
The WMO InterComparison of Radiation Codes used in Climate Models (ICRCCM) had already established the basis for evaluation of LBL codes, so MODTRAN was tested against these evaluations. Initial work by L. Kimball, L. Bernstein, and colleagues has now confirmed that with appropriate modifications (an improved radiance algorithm, implemented in MODTRAN3, and introduction of the mathematics to reduce spherical geometry to the "flat earth" approximation of ICRCCM), the IR spectrally integrated (0-3000 cm.sup.-1) cooling rates for H.sub.2 O, CO.sub.2, and O.sub.3 fall within the range of the LBL codes participating in the initial studies.
For two specific examples of a newer ICRCCM comparison (R. Ellingson, private communication) for MODTRAN3 and FASCODE; the measurements are from the AERI, a well-calibrated, surface-based interferometer developed by the Univ. of Wisconsin. In each case the results have been degraded to 10 cm.sup.-1 resolution and subsequently compared to the ICRCCM standard radiative transfer model: LBLRTM. While the statistics appear to slightly favor FASCODE over LBLRTM, in general (and for the 4 ICRCCM cases), LBLRTM and FASCODE are relatively indistinguishable, although the DOE-funded effort has made much progress in both computational efficiency and upgraded spectral sampling (resulting in improvements to better than 1%, not distinguishable on this scale. This agreement is not unreasonable in that both codes have a common lineage. The important aspect is that these two line-by-line codes (and the next generation, FASE) share the best agreement with the ICRCCM measurement set, approximately 2-3% mean difference and 4-6% rms difference (with values beyond 2400 cm.sup.-1 dropped because of extremely low S/N). The comparable statistics for MODTRAN are approximately twice as large, 5-10% mean and 6-12% rms. The difference in the statistical errors between MODTRAN and the line-by-line codes can also be seen in any direct comparison of their calculations, whether in transmittance or radiance; MODTRAN residuals are usually within a few percent of exact calculations, and every attempt is being made to reduce this magnitude.
Another recent example of validation of both FASCODE and MODTRAN also employs a Univ. of Wisconsin measurement set derived from their airborne instrument: HIS. A small spectral range (2000-2700 cm.sup.-1) of a measurement was taken from the NASA ER-2 over the Eastern Pacific. This spectral band has been chosen because it demonstrates the overlap in the thermal and solar Planck functions. The residual between measurement and FASCODE longward of 2400 cm.sup.-1 is due to reflected solar contributions. Putting in a minimum surface albedo of 0.05 and employing the MODTRAN solar capability reduces this residual between measurement and model almost completely. The virtue of having a single model with full thermal and solar radiation fields is quite apparent.
Given this success and general agreement with the LBL spectral cooling rate distributions for H.sub.2 O calculated by Clough et al., a more general energy deposition study has been undertaken. Again, the "flat earth" assumption has been maintained but otherwise a preliminary version of MODTRAN3 (missing only the new solar irradiance and the DISORT algorithm) was used for all calculations. The code was run from 0 to 50,000 cm.sup.-1 in two modes: (1) radiance with single scattering (which includes both thermal and solar sources), and (2) direct solar irradiance. A typical pair of up- and down-welling radiances for the full path at 15 km, might be measured from an aircraft flying at the tropical tropopause. The direct solar contribution is omitted and the assumed surface albedo is 0.1. Similar radiance calculations were done for 4 quadrature angles at each of 60 layer boundaries to approximate the 2.pi. fluxes while the approximations for daily averaged solar insolation were based upon a single solar incidence angle weighted for day of year (120) and latitude (tropical, 15N). An actual energy budget as determined from measurement, would also be the end result of complex sets of integrated data and spectral syntheses, no single instrument being able to provide the spectrally appropriate up- and down-welling contributions over the entire spectral range. Of course, this preliminary demonstration can only begin to provide estimates of sensitivities, even when validating data are available.
Characterization of surface properties from AVIRIS measurements is hampered by atmospheric attenuation and path radiances. MODTRAN, the Air Force PL/Geophysics Directorate moderate spectral resolution (2 cm.sup.-1) background radiance and transmission model, is often used to account for the atmospherics in AVIRIS measurements. It rapidly predicts the molecular and aerosol/cloud emissive and scattered contributions to observed radiances along with the atmospheric attenuation. MODTRAN has been extensively validated against both measurements and the high spectral resolution FASCODE model.
In view of the foregoing discussion, it is apparent that there remains an ongoing need to obtain refined estimates of atmospheric transmittance and background radiance, and that state-of-the-art methods are literally adapted for use almost as fast as they are developed by users that include the United States Air Force and other DOD agencies. The present invention is intended to satisfy that need.